A conventional method of making direct reduced iron is where iron ore or pellets which contain iron oxide are directly reduced using a reducing gas to obtain reduced iron. An example is a shaft furnace method represented by the Midrex process. In this type of method of making direct reduced iron, a reducing gas made from natural gas or the like, is forced into a shaft furnace from a tuyere located at the bottom portion thereof to reduce iron oxides, thereby obtaining reduced iron.
In recent years, of particular interest has been a process of manufacturing reduced iron in which a carbonaceous material, such as coal, is used as a reductant in place of natural gas. Such a method has already been put into practice and is referred to as an SL/RN method in which indurated pellets manufactured from iron ore are subjected to reduction through the application of heat using coal as a reductant.
Another reducing iron-making process is disclosed in U.S. Pat. No. 3,443,931, in which a mixture of pulverized iron ore and pulverized coal are agglomerated, and the agglomerated mass is subject to reduction through the application of heat on a rotary hearth, in high temperature atmosphere, yielding reduced iron.
Reduced iron obtained using the above-mentioned methods is charged into an electric furnace directly as source iron or in the form of briquettes. With the increasing trend of recycling scrap in recent years, this reduced iron is of particular interest, since it may be used as a diluent of impurities contained in the scrap.
A conventional method, however, does not involve separating slag components such as SiO.sub.2, Al.sub.2 O.sub.3, and CaO contained in the iron ore or the like and in the carbonaceous material (coal or the like), from the molten iron produced. Therefore, the resultant reduced iron has a relatively low iron content (iron purity of metallic iron). In actual practice, these slag components are separated and removed during a subsequent refining process. However, an increase in the amount of slag not only decreases yield of refined molten iron, but significantly increases the running cost of an electric furnace. Therefore, reduced iron is required to be iron rich and have a relatively low content of slag components. In order to meet this requirement, it is necessary for the above-mentioned conventional reducing iron-making methods to use iron-rich iron ore, which narrows the choice of source materials for making iron.
Furthermore, a goal of the conventional methods described above is to obtain a reduced solid product as an intermediate product in an iron making process. Therefore, additional steps such as conveyance, storage, forming briquettes, and cooling are required before reduced iron is sent to the next refining process. These steps involve a large energy loss, and a briquetting step requires excess energy and a special apparatus.
In addition, a smelting reduction process such as the DIOS method is known in which iron oxides are directly reduced to obtain molten iron. In this method, iron oxides are pre-reduced to an iron purity of approximately 30 to 50%, and then molten iron in an iron bath is subjected to a direct reducing reaction with carbon, to obtain metallic iron. However, this method has problems; since two steps are required, pre-reduction and final reduction within an iron bath, the work is complicated, and in addition, due to direct contact between molten iron oxide (FeO) present in an iron bath and the refractory of a furnace, the refractory is significantly damaged.
Japanese Patent Publication (kokoku) No. 56-19366 discloses a method in which an agglomerate of metal oxide, a solid carbonaceous material, and slag materials is reduced through the application of heat to thereby enclose reduced metal with slag shell while maintaining the shape of the agglomerate, and then the slag shell is melted to separate metal from slag. This method must generate a sufficient amount of slag to completely enclose reduced metallic iron in order to prevent the metallic iron from being re-oxidized. Thus, the slag materials content must be increased. Furthermore, this method is likely to generate slag having a relatively high FeO content, which raises a serious problem, in practical application, of significantly damaging the refractory lining of equipment.
Thus, it is quite important to realize a method of making metallic iron having a relatively low content of slag components, since such a method adds more value to a metallic iron product, reduces the running cost of an electric furnace, and provides a flexible choice of source materials.
Since slag having a relatively large iron oxide content melts refractory, it is very important for industrial feasibility of this kind of iron-making process to reduce the iron oxide content of slag, generated accompanyingly in a process of reduction, in order to minimize damage to refractory.